1. Field of the Invention
The present invention relates to thermoplastic polymers and methods for the crystallization of these polymers. More particularly, it relates to the discovery that the thermoplastic polymers may be expeditiously crystallized by the application of mechanical stress and heat to particles of the polymer without the hazard of stickiness and agglomeration of the polymer particles in the crystallizer.
2. Description of Prior Art
Crystallization of many thermoplastic polymers is necessary to make them workable in further processing steps toward the preparation of plastic end products. Crystallization is particularly important where the polymers are amorphous polymers, copolymers, or polymer blends which become sticky and agglomerate at relatively low temperatures. Without crystallization it is difficult to handle these polymer particles in blow molding, extrusion, and blending operations without problems of agglomeration of the polymer particles and sticking of the polymer to operating equipment. Polyesters of various types and copolymers containing polyesters usually must be crystallized before further processing, for example by xe2x80x9csolid statexe2x80x9d heating to increase melting points. Polyester particles tend to become sticky and agglomerate at temperatures much lower than their crystallization temperature, and are importantly typical of thermoplastic polymers in general.
Thermoplastic polymers have been crystallized by slowly heating the polymer particles through a sticky phase which occurs at temperatures somewhat lower than the crystallization temperature of the polymer. The polymer particles are slowly heated further to where the amorphous polymer chains are converted to crystals usually with the release of exothermic heat from the crystallization. It has been necessary to approach the exothermic phase slowly to prevent runaway temperature rises which can damage the physical properties and color of the polymers. With slow heating of the polymers long term elevated temperatures still can cause thermal damage to the polymers and yellowing of the product.
Even with slow heating using the present state of the art, parts of the thermoplastic particles become overheated, and semimelted so that the particles become sticky and agglomerate to form large masses in the crystallization equipment. This can damage the apparatus and the polymer properties, such as the intrinsic viscosities, melting points, and particle sizes.
The main prior art slow heat crystallization method requires the use of large vessel volumes per unit of production and the use of large amounts of inert gas, such as nitrogen, which is passed through the crystallizer to protect the hot thermoplastic from oxidation during the long hold up time required.
Tung et al in U.S. Pat. No. 5,919,872 disclosed that polyethylene terephthalate polyesters when coated with alkylene carbonates crystallize more quickly and at lower temperatures than the same uncoated polymers.
Herse et al in U.S. Pat. No. 5,663,290 disclosed crystallization of poly(ethylenenaphthalenedicarboxylate) and its copolymers by maintaining water content below a critical point throughout the crystallizing process so that a separate drying step is not required.
Keilert in U.S. Pat. No. 5,628,947 also disclosed a process for simultaneous drying and crystallization of crystallizable thermoplastics, by cooling strands of extruded polymer in 1.5 to 20 seconds.
Palmer in U.S. Pat. No. 6,344,539 discloses a process for crystallizing polyester granules providing a bi-component structure consisting of a thin crystalline skin and an amorphous interior. The crystalline skin prevents the granules from sticking in subsequent hot processing and the amorphous interior facilitates melting at lower temperatures in subsequent extrusion processes.
U.S. Pat. No. 3,544,523 to Mobil Oil teaches that polyester granules may be prevented from sticking by coating the granules with a small amount of anti-caking agent such as talc.
Al Ghatta et al in U.S. Pat. No. 5,714,571 disclose a method of continuous crystallization of polyester resin in a whirling fluid bed crystallizer where fluiding nitrogen gas enters at a temperature not lower than 195xc2x0 C. and the average residence time of the polymers is higher than 5 minutes. Feeding the polymer from the fluid bed to a mechanical mixer, which moves the polymer longitudinally submitting the polymer to radial mixing, is disclosed. The fluid bed disclosed uses non-backmix piston type continuous flow to obtain uniform crystallization values in the product. The fluidization velocities of the nitrogen gas through the fluid bed is between 3 and 5 meters per second. The crystallinity degree of the polymer coming out of the fluid bed treatment is between 38 and 42 percent and is brought to 40 to 50 percent by subsequent crystallization processes which are conveniently carried out by moving longitudinally through a mechanical mixer at temperatures 10 to 30xc2x0 C. higher than that of the polymer leaving the fluid bed. Another fluid bed may be used instead of a mechanical mixer.
Shelby et al in U.S. Pat. No. 6,159,406 disclose a process for introducing strain-induced crystallization to polyesters by sending amorphous polyesters directly from a melt phase reactor through a traditional strand die and then uniaxially stretching the extruded polymer strand resulting in an increased rate of crystallization than obtained with traditional thermal crystallization.
In summary, the prior art provides for the crystallization of thermoplastic polymers by: (1) an inert gas-fluidized bed of polymer particles with the use of large amounts of difficult to heat and expensive inert gas; (2) continuously stretching strands of polymer to increase the rate of thermal crystallization requiring a melt extruder and strand chopper at the site of the crystallization operation; (3) the use of a whirling fluid bed crystallizer heated by inert gas at more than 195xc2x0 C. at a velocity of 3 to 5 meters per second to obtain 38 to 42 percent crystallization requiring the operation of a sophisticated industrial installation; and the addition of costly property changing coatings to the polymers.
It is therefore an object of this invention to provide an effective, non-agglomerating, method of crystallizing crystallizable thermoplastic polymers in a simple mechanically fluidized crystallizer where the friction generated in the mechanical fluidization deforms the polymer particles to create sufficient stress in the polymer particles to orient the polymer molecules into crystals with the heat generated by the mechanical fluidization.
It is also an object to provide a method of crystallization that does not require the flow or heating of inert gases.
It is a further object to provide a method of crystallization that requires no additives.
It is a further object to provide a method which may be operated at locations remote from sophisticated industrial installations, not requiring melting, extrusion, stretching, quenching, chopping, drying, or solid state polymerizations of the polymers.
It is a further object to provide a method which allows the crystallization of the polymers accurately to the degree of crystallization desired.
It is a further object to provide a method which is particularly effective in crystallizing polyester polymers and copolymers which are difficult to crystallize without agglomeration.
I have now discovered that deformation of crystallizable thermoplastic polymer particles applied in a mechanically fluidized crystallizer creates heat of friction and mechanical stress on the particles sufficient to align the polymer molecules into crystallized polymers without agglomeration of the polymer, and I have also found that desired degrees of crystallinity may be obtained in the polymer particles by the accurate control of temperature and stress treatment time in a mechanically fluidized crystallizer. The discovery overcomes the limitations of the prior art and provides the objects of the invention with a practical method of crystallizing thermoplastic polymers in a controlled manner without agglomeration.
I have now discovered a method of accurately crystallizing particles of crystallizable amorphous thermo-plastic polymers in a mechanically fluidizing crystallizer which provides heat and mechanical stress to the polymers sufficient to crystallize the polymers without agglomeration of the polymer particles in the crystallizer. In the instant method, amorphous thermoplastic polymer particles are mechanically fluidized in a cylindrical crystallizer by means of fluidizing blades moving around, and in close proximity to, the inner perimeter wall of the crystallizer.
The fluidizing blades are moved around the inner perimeter wall of the crystallizer at speeds which cause mechanical friction between the polymer particles and the wall of the crystallizer, the fluidizing blades, and other polymer particles, with the friction providing heat and mechanical stress on the polymer particles by deforming them.
The mechanical stress must be maintained on the polymer particles at temperatures exceeding the crystallization temperature of the polymer until sufficient stress is provided to align the molecules of the polymer into crystal lattices, forming crystallized polymer particles without agglomeration of the particles in the crystallizer.
The severity of the mechanical deformation and the stress and heat created thereby may be regulated to provide the amount of crystallization required in the crystallized polymer particles.
Temperature control is required during crystallization and the fluidizing blades moving around, and in close proximity to, the inner perimeter wall, provide effective contact heat transfer between the wall of the crystallizer and the polymer particles for heating or cooling.
The heat provided by the friction of the blades usually amounts to a majority of the heat required to maintain the temperatures exceeding the crystallization temperatures of the polymers.
Although the crystallized polymer particles are sometimes advanced to further processing steps such as solid state polymerization and blow or injection molding, the crystallized particles are usually cooled to ambient temperature. The cooling may be done by contact cooling through the crystallizer wall batchwise or may be done in a separate cooling device either continuous or batchwise.
A variety of amorphous thermoplastic polymers may be crystallized by the instant method including polyesters, polyolefins, polycarbonates, polyamides, and polyphenylene sulfides.
The method is particularly useful where the amorphous thermoplastic polymers include biodegradable polyester copolymer blends, silicone polyester copolymers and liquid crystal polyester copolymers which exhibit softening temperatures lower than 100xc2x0 C.
It is preferred to carry out the instant method when the mechanical friction is maintained for a period of time between 20 and 200 minutes until sufficient mechanical stress is provided to the polymer particles at temperatures exceeding the crystallization temperature of the polymer by between 10 and 50xc2x0 C. When the minimum friction time and temperature were not provided, crystallization was insufficient to prevent sticking of the particles in subsequent operations. When the temperatures were exceeded, crystallization was achieved but some sticking occurred on the heat transfer walls of the crystallizer.
The crystallizer wall temperatures are preferably maintained within 30xc2x0 C. of the temperature of the polymer particles to assure effective heat transfer to or from the wall and to provide agglomerate-free crystallizer operation.
It is unnecessary to provide a fluidizing flow of inert gas to achieve excellent crystallization in the instant method, and it is possible to use the method allowing ambient air to be in the crystallizer during operation. It is preferred, particularly when working with polymers exhibiting high crystallization temperatures, to blanket the. crystallizer and the polymer particles contained therein with inert gas to exclude air and prevent oxidative degradation of the polymer particles during crystallization.
The instant method may be controlled so that a thin coating of crystallization is applied to the surface of the polymer particles where the subsequent treatments require only a small degree of crystallization to provide non-sticking subsequent operations with the polymer products. It is preferred to maintain the mechanical friction in the instant crystallization method until between 20 and 80 percent of the polymers are crystallized.
If more crystal content in the polymer particles is desired, the amount of crystallized polymer formed may be increased by increasing the time that the mechanical friction is maintained.
A variety of mechanically fluidized crystallizers may be used in the instant method. The method is preferably operated where the particles of amorphous thermoplastic polymers are mechanically fluidized in a horizontal cylindrical crystallizer by means of a plurality of fluidizing blades, each attached to a center horizontal shaft rotating so that the paths of the blades substantially cover the inner perimeter wall of the crystallizer.
A more preferable operation of the instant. method is where the horizontal cylindrical crystallizer is equipped, in addition to the fluidizing blades, with one or more high speed mixing blades spinning at the end of a shaft coming through the perimeter wall of the crystallizer, with the high speed blades turbulently agitating the fluided polymer particles to improve contact heat transfer in the crystallizer.
Best heat contact heat transfer is obtained where the high speed mixing blades exhibit lengths between 10 and 30 percent of the diameter of the cylindrical crystallizer and spins with blade tip speeds between 7 and 30 times faster than the linear speeds of the fluidizing blades.
The use of the instant method allows the preparation of a new crystallized thermoplastic polymer composition in which each particle is crystallized to substantially the same degree, between 20 and 80 percent crystals, and free of agglomerates.
The instant method may be used with a wide range of crystallizable thermoplastic polymers which exhibit quite different crystallization temperatures, the method is effective where the temperatures of operations exceeding the crystallization temperature are between 60 and 300xc2x0 C.
The method is particularly effective at remote sites distant from manufacturing plants and is there frequently operated batchwise on either large or small scales. Conversely, especially where large production rates are required, the method is operated where the amorphous thermoplastic polymer particles are continuously mechanically fluidized, stressed, and heated to form crystallized polymer particles without agglomeration of the polymer particles in the crystallizer.
Although all crystallizable thermoplastic polymers must be crystallized carefully, polyester polymer present the largest crystallization problem because the polyesters usually become sticky at temperatures lower than their crystallization temperatures. Also, the polyester and polyester derived polymers comprise one of the largest polymer markets where the copolymer particles must be crystallized frequently remote from their production site.
A stepwise method has been found for crystallizing amorphous polyester polymer particles in a mechanically fluidizing horizontal cylindrical crystallizer without stickiness in the crystallizer.
In the first step amorphous polyester polymer particles are mechanically fluidized in a horizontal cylindrical crystallizer by means of fluidizing blades moving around, and within between 1 and 30 millimeters of the inner perimeter wall of the crystallizer.
In the second step the fluidizing blades are moved around the perimeter wall of the crystallizer at speeds between 2 and 12 meters per second to cause mechanical friction between the polymer particles and the wall of the crystallizer, the fluidizing blades, and other polymer particles, the friction providing heat and mechanical stress to the polymer particles by deforming the particles until the stress orients the polymer molecules in the direction of the stress to form polyester polymer crystals.
The third step comprises adding heat or cooling as needed for the crystallization by contact heat transfer through the inner wall of the crystallizer to regulate the temperature of the wall of the crystallizer to within 30xc2x0 C. of the temperature of the particle wall.
In the fourth step the mechanical friction is maintained for a period of time between 5 and 200 minutes until sufficient mechanical friction is provided to the polymer particles to generate polymer particle temperatures exceeding the polymer crystallization temperature by between 10 and 50xc2x0 C. and sufficient stress is provided to orient the polymer molecules so that the particles contain between 20 and 80 percent crystals.
Finally the crystallized polyester polymer particles are cooled to ambient temperature.
The foregoing method is effective where the amorphous polyester polymer particles comprise between 5 and 50 percent copolymer.
To assure that no oxygen induced degradation occurs, the method is preferably operated where the horizontal cylindrical crystallizer contains nitrogen gas to exclude air from contact with the polyester polymer particles.
For high production rates of crystallized polymer particles, it is preferred to operate the horizontal cylindrical crystallizer in a continuous manner.
The horizontal cylindrical crystallizer may use a variety of fluidizing blades. To achieve efficient application of stress on the polymer particles and effective crystallization it is preferred that the fluidizing blades moving around and within between 1 and 30 millimeters of the inner perimeter wall of the crystallizer be formed in one of the following shapes: flat solid rectangles, flat solid triangles with their bases turned toward the crystallizer wall, curved solid rectangles, curved solid triangles, perforated shapes, and shovel shapes.
A wide variety of polyesters and their copolymers and blends may be crystallized effectively in the instant method. Amorphous polyester polymer particles which are preferably crystallized include polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyethylene naphthalene dicarboxylate, polyethylene isophthalate, and copolymers thereof.
When the method described in the foregoing paragraphs is used on amorphous polyester polymer particles, a new composition of polyester polymer particles, each substantially equally crystallized to between 20 and 80 percent crystal content and free of agglomerates, is formed.
Some of the terms used in this disclosure are defined as follows:
(1) Mechanical Stress means the strain created by the mechanical force exerted on one surface by another surface to deform or draw out the surfaces without breaking them.
(2) Mechanical Friction means the mechanical rubbing of one surface against another and the resistance to motion of. the surfaces that touch.
(3) Softening Temperature means the temperature where amorphous polymer particles soften and develop sticky surfaces.
(4) Crystallization Temperature means the temperature where amorphous polymer particles are converted to crystal form, usually determined in laboratory heating tests where changes in refractive indices of the polymers are observed.
(5) Degree of Crystallization is the portion of a polymer particle converted from amorphous to crystalline form measured by changes in refractive indices. Degree of crystallization may also be determined by use of a Densitometric Column.